Since you would like that device to activate two ways here is how to do it. You have already solve the one way buy running voltage through a simple switch as shown in one of the picture examples.
You now need for voltage to engage the device with the engine running. This is quite simple. Run a wire from the fused side or your dash radio fuse Or install an inline fuse and pick up power from your fuse block that is switched off with the ignition off. Then just run it to the side of the switch connected to the device. You now have an Either circuit. Either one will activate the device.
Yes I know turning the switch on with the engine off will send power back to the fuse panel but that can be solved easily with a diode inline with that line.
